The present invention relates to a corona charging device for depositing charge on an adjacent surface. More particularly, it is directed to a corona charging arrangement usable in a xerographic reproduction system for generating a flow of ions onto an adjacent imaging surface for altering or establishing an electrostatic charge thereon. Still more particularly, this invention is directed to improved configurations for corona discharge devices of the type disclosed in pending patent application Ser. No. 595,656 in the joint names of T. Davis and G. Safford and U.S. Pat. No. 4,057,723 in the joint names of D. Sarid and B. Springett, both applications being assigned to the assignee of this application.
In the electrophotographic reproducing arts, it is necessary to deposit a uniform electrostatic charge on an imaging surface, which charge is subsequently selectively dissipated by exposure to an information containing optical image to form an electrostatic latent image. The electrostatic latent image may then be developed and the developed image transferred to a support surface to form a final copy of the original document.
In addition to precharging the imaging surface of a xerographic system prior to exposure, corona devices are used to perform a variety of other functions in the xerographic process. For example, corona devices aid in the transfer of an electrostatic toner image from a reusable photoreceptor to a transfer member, the tacking and detacking of paper to the imaging member, the conditioning of the imaging surface prior, during, and after the deposition of toner thereon to improve the quality of the xerographic copy produced thereby. Both d.c. (d.c. potential connected to the coronode) and a.c. (a.c. potential connected to the coronode) type corona devices are used to perform many of the above functions.
The conventional form of corona discharge device for use in reproduction systems of the above type is shown generally in U.S. Pat. No. 2,836,725 in which a conductive corona electrode in the form of an elongated wire is connected to a corona generating d.c. voltage. The wire is partially surrounded by a conductive shield which is usually electrically grounded. The surface to be charged is spaced from the wire on the side opposite the shield and is mounted on a grounded substrate. Alternately, a corona device of the above type may be biased in a manner taught in U.S. Pat. No. 2,879,395 wherein an a.c. corona generating potential is applied to the conductive wire electrode and a d.c. potential is applied to the conductive shield partially surrounding the electrode to regulate the flow of ions from the electrode to the surface to be charged. Other biasing arrangements are known in the prior art and will not be discussed in great detail herein.
Several problems have been historically associated with such corona devices. One major problem has been their inability to deposit a relatively uniform negative charge on an imaging surface. Another problem has been the growth of chemical compounds on the coronode which eventually degrades the operation of the corona device. Yet another problem has been the degradation in charging output resulting from toner accumulations on the coronode and surrounding shield structure. One still further problem is wire vibration which leads to arcing and wire fracture. These problems, among others, are specifically addressed in the aforementioned applications in which there are proposed novel corona discharge configurations which substantially reduce or alleviate the problems noted above, and other problems associated with prior art corona devices, as is discussed more fully therein.
By way of summary, the aforementioned application Ser. No. 595,656 discloses a novel corona device for use in electrostatic reproduction machines which comprise a corona discharge wire coated with a relatively thick dielectric coating, the thickness of the coating being sufficient to prevent the flow of conduction current from the wire. Generation of charge is accomplished by means of a voltage at the dielectric surface established by capacitative coupling through the dielectric material. The magnitude of the flow of charge to the surface to be charged is regulated by the application of a d.c. bias potential to a conductive shield adjacent the electrode.
U.S. Pat. No. 4,057,723 proposes a modification of the structure of Ser. No. 595,656 whereby an exponential rise in charging current is obtained by supporting the dielectric coated wire contiguous or very near the biased shield.
While the corona devices disclosed in the above noted applications individually solve many problems associated with other known corona devices, it is desirable to provide a corona device which retains the better characteristics of each design. For example, while the device of Ser. No. 595,656 has a uniformity of charge characteristics which is superior to prior art devices, it suffers from the same prior art disadvantages associated with a suspended coronode such as fragility in handling and vibration during operation. On the other hand, while the device of U.S. Pat. No. 4,057,723 exhibits a great degree of ruggedness, it has a less uniform charge output.